Noise control circuit



April 20, 1943.

B. TREvoR 24,316,902

NOISE CONTROL CIRCUIT Filed oct. 22. 1941' lNvENToR m frei/0r MVM,

Patented Apr. 20, 1943 NOISE CONTROL CIRCUIT Bertram Trevor, Riverhead,N. Y., assigner to Radio Corporation of America, a corporation ofDelaware Application October ,22, 1941, Serial No. 416,027

y(Cl. Z50- 20) Claims.

My present invention relates to a noise control circuit for a receiverof the angular velocityinodulated carrier wave type, and moreparticularly to a circuit for automatically silencing such receiver inthe absence of a carrier magnitude above a predetermined value.

It has been found in the past that when radio receivers are operatedwith automatic gain control (AGC), noise reproduction is greatlyincreased when rthe carrier amplitude drops below a predeterminedmagnitude. Hence, automatic silencing networks have been provided toprevent such noise reproduction when the carrier amplitude decreases toa low level which is insufficient to actuate the AGC circuit to reducethe sensitivity of the controlled pre-demodulator stages. In the case ofangular velocity-modulated carrier Wave receivers, and particularlythose of the phase modulation type, utilization of AGC circuits alsogive rise to the problem of noise reproduction in the absence of acarrier of predetermined amplitude. Operation with a phase modulatedcarrier wave (PM) receiver in the 40G-500 megacycle (mc.) range hasindicated that automatic noise silencing is required if AGC is to beemployed. This is particularly true in the case of a PM receiveroperating as a relaying device. In an ultra-high frequency relayreceiver of the PM type the noise output of the receiver, in the absenceof a carrier of sufcient amplitude, is high enough to actuate aremotecontrol stop-start relay.

Accordingly, it may be stated that it is one of the main objects of mypresent invention to provide in a receiver of angular velocity-modulatedcarrier Waves having an AGC circuit, a device responsive to apredetermined decrease of the carrier amplitude for automaticallypreventing transmission of electrical impulses to thediscriminator-rectier employed in the receiver for demodulationpurposes.

A more specic object 0f the invention may be stated to reside in theprovision of an ultrahigh frequency PM receiver which employs a devicefor automatically rendering the amplitude modulation limiter thereofineffective when the carrier amplitude decreases below a predeterminedtolerable level.

Still another object of my invention is to provide in a PM receiveroperating in the 400-500 mc. band an AGC circuit which inherently causesan increase in noise output of the receiver in response to a decrease ofthe carrier amplitude below a desired level, there being additionallyprovided a noise control circuit which is responsive to the deviceactuating the AGC circuit for preventing transmission of electricalimpulses to the receiver discriminator-rectier when such carrieramplitude decreases occur.

Still other objects of my invention are to .improve generally theefficiency and reliability of ultra-high frequency receivers operatingas relay receivers, and to provide such efcient and reliable operationin an economical manner.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have in-' dicateddiagtammatically a circuit organization whereby my invention may becarried into effect.

Referring now, to the accompanying drawing wherein are shown only thosecircuits of a PM receiver o f the ultra-high frequency type which areessential to a proper understanding of the invention, it will beunderstood that the tuned Circuit l is the output circuit of anintermediate frequency (I. F.) amplifier. The amplifier Amay compriseone or more I. F. tube stages. These I. F. stages may be preceded by anytype of converter Vstage which is capable of vreducing the frequency o fthe collected ultra-high frequency waves to the operating I. F., in thiscase a value of 8 mc. As stated previously, in the specific case beingconsidered in this application the PM Waves are in the 400-500 marange.Specically the center frequency of the collected waves is 402 mc, The PMwaves themselves may be wide band or narrow band with respect to carrierdeviation.

As is Well known to those skilled `in the art, the diiference betweenthe PM and FM waves resides in the fact that the former includes a widerfrequency deviation `at the higher modulation frequencies than in thecase of the latter. The modulation amplitude is translated into aproportional carrier frequency deviation at the transmitter, while therate of deviation corresponds to the modulation frequencies per se. Thesecondary circuit 2 is tuned to the operating I. F. value, and isreactively coupled to the primary circuit l so -as'to provide the proper`pass band width to pass the entire carrier frequency deviation of thePM waves.

The `amplitude modulation effects which may exist on the carrier waveare eliminated vby a limiter tube 3. rlhese amplitude modulation effeatsare Caused ybvffadine..noise impulses and known to those skilled in theart, the limiter` circuit is designed s as to have an input-outputcharacteristic which is substantiallyA horizontal beyond a predeterminedinput energy level.v

The tube functions in the manner of a saturated amplifier, and in thatway feeds to the subsequent demodulator, or discriminator-rectifien aPMv wave which is of substantially constant carrier amplitude and freeof any amplitude modulation eifects. The discriminator-rectier circuitmay be constructed in any well known manner, and it is not believednecessary to describe such a circuit. In general, it functions toconvert the PM wave into the modulation frequencies of variableamplitude, and the latter arev utilized to actuate a modulationfrequency utilization circuit. Such latter circuit may be a remotecontrol stop-start relay, printers, facsimile equipment, and the like.

AGC is provided for the pre-limiter networks; that is, for theultra-high frequency amplifier and I. F. amplier tubes. Furthermore, itis desired to provide a source of amplitude modulation voltage toenergize a monitor so that the receiver operator may be able to observefor himself when ampli-v tude modulation exists on the received carrierwave. Both these functions are accomplished by connecting the anode andcathode of a diode rectifier 6 across the tuned circuit 2. The anode isconnected to the high potential Vside of input circuit 2 while thecathodeis connected to the low potential side through condenser 1. Thelow potential terminal of condenser 1 is connected t0 ground through acondenser 8 having a magnitude of approximately 0.1 microfarad (mi.) .YThe junction of condensers 1 and 8 may be connected to a source ofnegative bias, establishing for example a lmagnitude of 105 volts,through a resistor 9 of approximately 100,000 ohms.

The load resistor of rectifier 6 is designated by numeral l0, and theresistance thereof may be approximately 10,000 ohms. VOne end of theresistor is connected to the cathode 0f diode'l through filter coil Il,Awhile the other end ofA the resistor l0 is connected to the junction ofcondensers 1, 8 and I2'through a milliammeter I which may indicate arange of O'to 1 milliampere. VCondensers 1 and l2 in combination withcoil ll form a low pass filter toprevent I. F. currents from reachingthe Vload resistor Il).V Y The direct current voltage developed acrossresistor l0 is utilized for AGC purposes, andv alsoV to actuate thenoise silencing tube 20. The control grid circuits of the controlledtubes are connected through a lter resistor I3 to the cathode -end ofresistor l0. Since that end ofA resistor I0 becomes increasinglypositive in polarity with an increase in carrier amplitude, somepolarity reversing device is utilized in the AGC lead to the controlledtubes. In the absence of received carrier waves, or when the receivedcarrier amplitude is below a predetermined intensity level, the directcurrent voltage developed across resistorll is then in approximateproportion 'to 'the carrier amplitude and is very small. Accordingly,the AGC bias for the controlled tubes is essentially zero with theresult that the sensitivity of the controlled tubes is at its maximum.

To prevent the noise impulses from being transmitted to thediscriminator-rectifler, whenV the carrier decreases below apredetermined amplitude, there is provided the noise control tube whichmay be of the 6J 5 type. The cathode 2| of this tube is connectedthrough resistor 22 to the -105 volt direct current source, and theresistor 22 may have a magnitude of approximately 2500 ohms. The controlgrid of tube 28 is connected by resistor 23 to the cathode end ofresistor I0, and resistor 23 may have a magnitude of approximately100,000 ohms. The cathode end of resistor 22 is connected to thepositive terminal of the direct current voltage source through a re.sistor 24 Vhaving a magnitude of approximately 35,000 ohms, and theplate 25 is connected to the grounded positive terminal of the aforesaidsource through the winding 26 ofthe relay 21. The relay 21 isschematically represented, and is shown enclosed by dotted lines. vSincethose skilled in the art are fully yaware of the construction of a relayof the type shown in 21, it is'not schematic appearance thereof.

As is well known to those skilled in the art,

the relay Winding 26 energizes an electro-magnet which functions toattract the armature 28.V The latter is normally in contact with thecontact point` 30 with normal signal level input to the receiver. Thespring 3l will pull the armature 28 away from circuit-closing contactwith point 30 when the current new through winding 26 decreases below adesired intensity. The pivoted end of armature 28 is connected to asource of positive voltage, as for example +250 volts, and the contactpoint 30 is connected by lead 40 to the screen grid 3 of limiter tube 3.Lead 40 is connected to the screen gridV through a filter networkcomprising series resistors 50 and shunt condensers 5I. .When thecurrent flow through relay winding 26 is of minimum value, the spring 3|is able to pull armature 28 out of contact with contact point 30 therebybreaking the positive supply voltage to the screen grid 3. This causesthe limiter tube to be ineffective to transmit any electrical impulsesto the subsequent discriminator-rectifier. On the other hand when thespace current-flow through tube V2|) rises to an intensity such thatwinding 26 energizes its electro-magnet to pull armature 28 into contactwith point 30, the screen grid 3 will be energized and the limitertubeWill be of normal transmission eiliciency.

through circuit 2 there will be substantially no Vdirect current voltageacross resistor l0, the diode load resistor. As soon asI; F. energy4` isapplied to 2 there will flow in resistor Jl0`a rectified current givinga Vdirect current potenti'alvacrossY this resistor, the positive endof`which is connected to the (grid of 20 through resistor 23. Thecathode of 20 connects to the voltagedivider 22,'24 which puts thecathodeat a positive potential4 with respect to the negative 'side ofthe plate supply. -Since no direct current flows through eitherresistor-'9 or`23, it can bestated that the bias between thegrid andcathode'of 20 is equal to thevolta'ge` across resistor lllim'inus thevoltageacro'ss Vresistor '22. VThe circuit values are such that this.bias is always'negative so thatino grid current iiowsin tube 20;"A .L

It is to be noted that the ground on the positive side of the platebattery in no Way affects the operation of tube 20, since this one pointis only ground for direct potentials for the entire circuit drawn. Thereason for grounding the positive side of the battery is to allow theautomatic gain control circuits to function properly. In View of whathas been said above, it should be clear that a decrease of I. F. inputat 2 will cause an increase in bias on tube 20, since the voltage acrossI acts in a direction to decrease this bias. An increase in bias causeslower current to flow in relay 26 thereby opening its contacts. Theaction of the automatic gain control circuit is to maintainsubstantially constant I. F. input to 2, which, in turn, causes asubstantially constant potential lacross IIJ for all signal levels abovethe receiver noise. In order for the noise silencing circuit tov operateproperly it is necessary that the diode current, at full receiver gainand no signal input, be considerably less than normal diode current withsignal ap plied. Thus, the AGC circuit cannot raise the gain of thereceiver with no-signal input to such a high value that the receiverinternal noise causes the diode current to come up to its normaloperating value. The maximum receiver gain may be limited to the correctvalue by adjusting either the screen potentials, or self-bias resistors,or by any other convenient means. A

The receiver is normally operated with the AGC circuit active, and withno carrier present the receiver noise level is brought up enough to givea diodecurrent of approximately 0.2 milliampere (ma). about 0.7 ma. sothat a current diierential of 0.5 ma. owing through the 10,000 ohmresistor IU produces a voltage differential of 5 volts. This is suicientto control the grid of tube 20, and in turn operates the relay 21 in itsplate circuit. Under normal conditions the relay contacts 30 and 28 areclosed thereby supplying the proper positive potential to the screengrid of the limiter tube 3. When the relay contacts are open, however,the limiter tube becomes inactive thereby cutting oi all noise impulseswhich would otherwise lbe transmitted to the audio utilization circuit.

A pair of amplitude modulation monitor terminals B0 may be coupled tothe ends of winding 25 by means of the coupling condenser 6l having avalue of 0.1 mf. The function of these monitor terminals is to permit ameasurement t0 be made to determine the amount of amplitude modulationexisting on the received carrier. The function of network 'l-l l-I2 isthat of a low pass filter, to prevent I. F. energy from appearing at theload resistor I0. The capacitor 8 serves only to ground the lowpotential side of the diode circuit for I. F. and audio voltages.Network SI1-5I is the usual I. F. filter arrangement which prevents I.F. energy from passing between tube 3 and lead 40.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made Without departingfrom the scope of my invention, as set forth in the appended claims.

What I lclaim is:

1. In an angular velocity-modulated carrier wave receiver of the typeincluding an amplitude modulation limiter tube adapted to feed a follow-Normally, the diode current is ing discriminator-rectier; an improvementcomprising a direct current circuit energizing an electrode of saidlimiter tube to render it operative for substantially minimizingamplitude modulation effects which may exist on the received carrier,and additional means responsive to a de# crease in the received carrieramplitude below a predetermined value for rendering said energizingcircuitl ineiective thereby to render said limiter ineffective totransmit any electrical impulses.v i

2. In ycombination in an angular velocity-modulated carrier wavereceiverincluding'an amplitude modulation limiter tube adapted to-feed afollowing discriminator-rectifier, direct current means energizing apositive electrode of -said limiter tube to render it operative forsubstantially minimizing amplitude modulation effects which may exist onthe received carrier, additional means responsive to a decrease in thereceived carrier amplitude below a predetermined value for impairingsaid energizing means thereby to render said limiter ineffective totransmit any electrical impulses, and additional means'- responsive torelatively slow carrier amplitude variation for automaticallycontrolling the sensitivity of stages of said receiver prior to saidlimiter tube.

3. In combination in an angular velocity-modulated carrier wave receiverincluding an amplitude modulation limiter tube adapted to feed afollowing discriminatorrectier, a direct current circuit for energizinga positive electrode of said limiter tube to render it operative forsubstantially minimizing amplitude modulation effects which may exist onthe received carrier, additional means responsive to a decrease in thereceived carrier amplitude below a predetermined value for opening saiddirect current circuit thereby to render said limiter ineffective totransmit any electrical impulses, said last means comprising a rectinercoupled to the limiter tube input electrodes, an electron discharge tubehaving an input electrode responsive to the rectified output of saidrectier, and means in the output circuit f of said electron dischargetube for controlling said limiter tube energizing circuit.

4. In combination in a phase modulation receiver provided with a limitertube having input and output circuits, a rectifier coupled to thelimiter tube input circuit, and means responsive to the rectifiedvoltage output of said rectifier for opening or closing the energizationcircuit of at least one electrode of said limiter tube, said oneelectrode consisting of the screen grid electrode.

5. In a phase modulation receiver of the type operating in theultra-high frequency range and including a limiter tube provided withinput and output electrodes and a screen grid electrode, a directcurrent circuit for keping said screen grid at a positive voltage, arectifier coupled to the limiter tube input electrodes for deriving fromrelatively slow carrier amplitude variation a proportionally variabledirect current voltage, means for controlling the gain of the receiverin response to said voltage, and additional means responsive to saidvoltage for controlling the limiter screen grid energization circuit.

6. In combination in an angular velocity-modulated carrier wave receiverincluding an amplitude modulation limiter tube of the screen grid typeadapted to feed a following discriminatorrectiler, means energizing thescreen grid of said limiter tube to render it operative forsubstantially minimizing amplitude modulation effects which may exist onthe received carrier, and additional means responsive to a decrease inthe received carrier amplitude below a predetermined value for renderingsaid energizing means inoperative thereby de-energizing the screen gridof the limiter tube thereby to render said limiter ineiective totransmit any electrical impulses. 7. In combination in an angularvelocity-modulated carrier Wave receiver including an amplitudemodulation limiter tube adapted to feed a followingdiscriminator-rectier, said tube having at least one'ipositiveelectrode, means energizing said positive'electrode of said limiter tubeAto render it operative for .substantially minimizing amplitudemodulation veiiects which may exist on-the received carrier, relay meansresponsive to a decrease in the received carrier amplitude below apredetermined value for rendering said energizing ymeans inoperativethereby to render said limiter ineffective to transmit any electricalimpulses, and gain control means responsive to relatively slow carrieramplitude variation for automatically Icontrolling the sensitivity ofstages of said receiver prior to said limiter' tube.

8. In combination in anangular Velocity-,modulated carrier wave receiverincluding an amplitude modulation limiter tube adapted to feed afollowing discriminator-rectier, said limiter tube having at least onepositive electrode, a direct current energizing circuit for energizingsaid positive electrode ofY said limiter tube to render it operative forsubstantially minimizing amplitude modulation effects which may exist onthe recived carrier, additional means responsive to a decrease in thereceived carrier amplitude below a predetermined .value'ior renderingsaid limiter ineffective to transmit any electrical impulses, said lastmeans comprising a rectier coupled to the limiter tube input electrodes,an electron dis.- charge tube having an input electrode responsive tothe rectified output of said rectiiier, and a relay in the Youtputcircuit of saidV electrondischarge tube for controlling said limitertube energizing circuit. I l

9. In a phase modulation receiver of the type operating in the 400 to500 megacycle range and including a limiter tube provided with input andoutput electrodes and a screen grid electrode, a direct currentenergizing circuit for the screen electrode, a rectiiier coupled totherlimiter tube input electrodes Ifor deriving from relatively slowcarrier amplitude variation a proportionallylvariable direct currentvoltage, means for controlling the gain of the receiver in response tosaid voltage, and a relay responsive to said voltage for controlling thelimiter screen grid energizaticn circuit.

10. In a phase modulation receiver of the type including a. limiter tubeprovided with input and output electrodes and a screen grid electrode, arectier coupled to the limiter tube input electrodes for deriving fromrelatively slow carrier amplitude variation a proportionally variabledirect current voltage, means for eontrollingthe gain of the receiver inresponse to said voltage, and an electromagnetic relay means responsiveto said voltage for controlling the limiter screen grid energization.

BERTRAM TREVOR.

